Declare extern array for roll function names

Author: Omartech312

src/data.table.h

@@ -259,6 +259,7 @@ typedef enum { // adding rolling functions here and in frollfunR in frollR.c
   VAR = 6,
   SD = 7
 } rollfun_t;
+extern const char *const rfunNames[]; // Array of roll function names defined in froll.c
 // froll.c
 void frollfun(rollfun_t rfun, unsigned int algo, const double *x, uint64_t nx, ans_t *ans, int k, int align, double fill, bool narm, int hasnf, bool verbose, bool par);
 void frollmeanFast(const double *x, uint64_t nx, ans_t *ans, int k, double fill, bool narm, int hasnf, bool verbose);